1. Field of the Invention
The present invention relates to a polishing apparatus and a polishing method for polishing a peripheral portion of a substrate, such as a semiconductor wafer, and more particularly to a polishing apparatus and a polishing method for polishing a peripheral portion of a substrate by pressing a polishing tape against the peripheral portion of the substrate.
2. Description of the Related Art
From a viewpoint of improving yield in fabrication of semiconductor devices, management of surface conditions of a peripheral portion of a substrate has been attracting attention in recent years. In the fabrication process of the semiconductor devices, various materials are deposited on a silicon wafer to form a multilayer structure. As a result, unwanted films and roughened surface are formed on a peripheral portion of the substrate. It has been a recent trend to transport the substrate by holding only its peripheral portion using arms. Under such circumstances, the unwanted films remaining on the peripheral portion would be peeled off during various processes and could adhere to devices, causing lowered yield. Thus, in order to remove the unwanted films, the peripheral portion of the substrate is polished using a polishing apparatus.
This type of polishing apparatus polishes the peripheral portion of the substrate by bringing a polishing surface of a polishing tape into sliding contact with the peripheral portion of the substrate. In this specification, the peripheral portion is defined as a region including a bevel portion which is the outermost portion of the substrate and a top edge portion and bottom edge portion located radially inwardly of the bevel portion.
FIG. 1A and FIG. 1B are enlarged cross-sectional views each showing a peripheral portion of a substrate. More specifically, FIG. 1A shows a cross-sectional view of a so-called straight-type substrate, and FIG. 1B shows a cross-sectional view of a so-called round-type substrate. In the substrate W shown in FIG. 1A, the bevel portion is an outermost circumferential surface of the substrate W (indicated by a symbol B) that is constituted by an upper slope (an upper bevel portion) P, a lower slope (a lower bevel portion) Q, and a side portion (an apex) R. In the substrate W shown in FIG. 1B, the bevel portion is a portion B having a curved cross section and forming an outermost circumferential surface of the substrate W. The top edge portion is a flat portion E1 located radially inwardly of the bevel portion B and located radially outwardly of a region D where devices are formed. The bottom edge portion is a flat portion E2 located opposite the top edge portion and located radially inwardly of the bevel portion B. These top edge portion E1 and bottom edge portion E2 may be collectively referred to as near-edge portions.
In the conventional polishing apparatus, the polishing tape is pressed by a polishing head against the peripheral portion of the substrate to thereby polish the peripheral portion (for example, see Japanese laid-open patent publication No. 2002-126981). As shown in FIG. 2, when polishing the top edge portion of the substrate, a polishing tape 301 is pressed by a polishing head 300, with the polishing head 300 and the polishing tape 301 inclined.
However, polishing of the peripheral portion of the substrate with the inclined polishing tape results in an oblique edge surface of a device layer, as shown in FIG. 3. The device layer having such an oblique edge surface could raise the following problem in fabrication processes of SOI (Silicon on Insulator) substrate. The SOI substrate is fabricated by sticking a device substrate and a silicon substrate together. More specifically, as shown in FIG. 4A and FIG. 4B, the device substrate W1 and the silicon substrate W2 are stuck together, and then as shown in FIG. 4C, the device substrate W1 is ground from behind by a grinder, whereby the SOI substrate as shown in FIG. 4D is obtained.
Since the device layer has the oblique edge surface, an acute edge is formed as shown in FIG. 4D. Such an acute edge is easily broken, and fragments thereof may be attached as particles to a surface of the device layer. These particles on the device layer would cause defects in devices, thus lowering yield.
The Japanese laid-open patent publication No. 8-97111 discloses a polishing apparatus having a right-angled member that presses a polishing tape against a peripheral portion of a substrate. However, since the polishing tape has a certain thickness and a certain hardness, the polishing tape is not bent at a right angle along the right-angled member in a microscopic level, and the polishing tape is rounded to some degree. As a result, the oblique edge surface is formed on the device layer.
Further, due to a polishing load on the substrate through the polishing tape, the substrate may be bent or the position of the polishing tape may be changed during polishing. As a result, an edge surface of the device layer may be polished obliquely. The Japanese laid-open patent publication No. 2009-208214 discloses a polishing apparatus capable of keeping the substrate in an initial position by balancing a pressing force of a liquid supplied from a periphery supporting mechanism and a pressing force of a polishing mechanism. However, this periphery supporting mechanism is arranged in a position corresponding to the polishing tape and is designed to return the substrate W to its initial position by adjusting their mutual pressing forces when these pressing forces are unbalanced. With this mechanism, the substrate W may not be polished uniformly. As a result, the edge surface of the device layer may not be polished vertically.